C&GS Twelve-inch Theodolite

This double image shows a crew using a 12-inch theodolite and illustrates
two important roles in the surveying process: the observer (looking through
the theodolite) and the recorder (writing down the observer's angles). This
type of theodolite was used from the late 1800s until around 1930.

This image shows a crew and 12-inch theodolite and protective tent set
up at station ARMORY in New York, during the 1925 observing
season. In
the top portion of the image, the recorder enters the angles
as the observer reads and calls them, mentally doing calculations as a quick
error check on the observations. In the bottom part of the image,
the observer reads one of the instrument's four micrometer microscopes.

The 12-inch theodolite being used in the photo was considered highly accurate,
but slow to operate because of the number of microscopes
that had to be read. In order to compensate for flaws in the construction
of the circle itself, readings were taken at four evenly spaced positions
on the circle and averaged.

Setting a theodolite over a survey mark involved
more than just plunking the instrument down. First, a stable tripod had to be set
up or a stand of wood or steel had to be built to the necessary height - sometimes
more than 100 feet. Tents and umbrellas were necessary to protect the
instrument (not the observers!) from sun and wind. Once attached to
its stand, the theodolite was carefully plumbed over the center of the mark
and leveled. One crewmember observed while the other recorded the angles
as the observer called them out. For an interesting, and possibly surprising,
view of the location of the crew at ARMORY, look at the front-right
corner tower of the building shown here.

Historic Reference

The Coast Survey purchased theodolites and transits (which are much like
theodolites, but read somewhat differently), from various well-known makers. Instruments
bearing the names of Würdemann, Casella, Fauth, Gambey, and Gurley,
among others, saw increasing service as projects expanded across the continent.

The purchase of equipment was an expensive problem. In the Coast
and Geodetic Survey Annual Report of 1929, the Director wrote that the type
of equipment used by the agency was often very specialized and only needed
in limited quantities, making it difficult to develop, as well as extremely
expensive when obtained from private firms. He noted that after a
bid of $4,000 was received for one such apparatus, the instrument
division constructed it themselves for a total cost of $449.98!

Fortunately,
the Coast Survey has never lacked for skilled designers and
builders. One
such designer was Ernest G. Fischer, who in the mid-1890s designed a 12-inch,
three-microscope theodolite that bears his name.

Instrument Shown: 12-inch theodolite built by
Coast Survey

Location: New York

Manufacture Date: Late 1800s or early 1900s

Dates of Use: Late 1800s until around 1930

Photo Date: 1925

Works Consulted

Bowie, W. (1928). The Triangulation of the United States. Copy
located of part 1 of a two part journal article filed at
National Geodetic Survey. Journal unknown.

U.S. Coast and Geodetic Survey. (1929). Annual Report of the
Director, United States Coast and Geodetic Survey, to the Secretary of Commerce
for Fiscal Year Ended June 30, 1929. [Electronic version]. p 15.